Recently there has been much interest in the interaction of pathways descending from the brain with neurons of the intrinsic circuitry of the spinal cord involved with reflex movements, interlimb coordination, and ascending pathways. This proposal aims at investigating relationships between descending pathways, one of the spinal cord coordinating systems (the long descending propriospinal tract-LDPT), and peripheral afferents. An investigation of this interaction has not been attempted previously. This proposal grew from an anatomical study of the location of the cell bodies of origin of the LDPT. The first phase of this proposal involves a single unit analysis with glass micropipettes of the sensory input to the cells or origin of the LDPT in order to find out more about this tract. This analysis will utilize electrical stimulation of forelimb nerves to determine whether synaptic connections are mono- or polysynaptic, and natural stimulation of the forelimb to determine modalities of sensory inputs and sizes of peripheral fields. The second phase will use the effects of L-DOPA and other adrenergic drugs to "open up" the neuronal circuits in order to observe the peripheral input of LDPT neurons under conditions which more closely approximate those in which locomotion takes place. In the third phase the function of single LDPT neurons will be observed during "fictive" locomotion while stimulating one of the locomotor regions. It will be possible to correlate the type of LDPT neuron, its peripheral field, and the phase of locomotion during which it is active.